Ink jet printer with improved dry time

ABSTRACT

An ink jet print head includes an array of nozzles, each nozzle being capable of selectively producing when actuated at least two ink drop sizes including a larger ink drop size and a smaller ink drop size. In response to a pixel density signal of maximum density value at a respective pixel location, a nozzle prints a drop of the larger ink drop size at the respective pixel location on a reference raster and the same or a different nozzle prints a drop of a smaller drop size at a pixel location adjacent to the respective pixel location on a shifted raster to provide for full coverage with improved dry time capability of the printed ink drops.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to the following U.S. patent applicationsfiled on even date herewith:

U.S. application Ser. No. 09/940,224 filed in the names of James Newkirket al and entitled “METHOD AND APPARATUS FOR INCREASING NUMBER OFAVAILABLE PRINTING GRADATIONS ON AN INK JET PRINTER;” and

U.S. application Ser. No. 09/940,195 filed in the names of Rodney L.Miller et al. and entitled “METHOD AND APPARATUS OF OPTIMIZING DISCRETEDROP VOLUMES FOR MULTIDROP CAPABLE INKJET PRINTERS.”

FIELD OF THE INVENTION

The invention relates generally to the field of ink jet printing andmore particularly to ink jet printing with a print head capable ofdepositing multiple drop sizes.

BACKGROUND OF THE INVENTION

Ink jet printing is a non-impact method for producing images by thedeposition of ink droplets in a pixel-by-pixel manner onto an imagerecording element in response to digital signals. There are variousmethods which may be utilized to control the deposition of ink dropletson the recording element to yield the desired image. In one process,known as drop-on-demand ink jet printing, individual ink droplets areprojected as needed onto the recording element to form the desiredimage. Common methods of controlling the projection of ink droplets indrop-on-demand printing include piezoelectric transducers and thermalbubble formation using heat actuators. With regard to heat actuators, aheater placed at a convenient location within the nozzle or at thenozzle opening heats the ink in selected nozzles and causes a drop to beejected to the recording medium in those nozzles selected in accordancewith image data. With respect to piezoelectro-actuators, piezoelectricmaterial is used which piezoelectric material possesses the propertysuch that when electrical field is applied to the material a mechanicalstresse is induced therein reducing the volume of the nozzle and causinga drop to be selectively ejected from the nozzle selected. The imagedata applied to the print head determines which of the nozzles areselected for ejection of a respective drop from each nozzle at aparticular pixel location on a receiver sheet. Some drop-on-demand inkjet printers described in the patent literature use both piezoelectricactuators and heat actuators.

In another process, known as continuous ink jet printing, a continuousstream of droplets is charged and deflected in an imagewise manner ontothe surface of the recording element, while unimaged droplets are caughtand returned to an ink sump. Ink jet printers have found broadapplications across markets ranging from desktop document and pictorialimaging to short run printing and industrial labeling.

A typical ink jet printer reproduces an image by ejecting small drops ofink from a print head containing an array of spaced apart nozzles, wherethe ink drops land on a receiver medium (typically paper) to form roundink dots. In some printers, all drops are the same size, and therefore,all dots are the same size. Normally, these drops are deposited withtheir respective dot centers on a rectilinear grid, a raster, with equalspacing, p, in the horizontal and vertical directions. Therefore, toachieve full coverage of the ink it is necessary for the dots, 10, tohave at least diameter p*sqrt(2), as shown in FIG. 3. Some printerdesigns may allow for even bigger dots in order to compensate forunwanted variations in the placement of the drops.

Modem inkjet printers may also possess the ability to vary (over somerange) the amount of ink that is deposited at a given location on thepage. Ink jet printers with this capability are referred to as“multitone” or gray scale ink jet printers because they can producemultiple density tones at each pixel location on the page. Somemultitone ink jet printers achieve this by varying the volume of the inkdrop produced by the nozzle by changing the electrical signals sent tothe nozzle or by varying the diameter of the nozzle. See for exampleU.S. Pat. No. 4,746,935. Other multitone ink jet printers produce avariable number of smaller, fixed size droplets that are ejected by thenozzle (or by plural nozzles during different passes of the nozzlearray), all of which are intended to merge and land at the same pixellocation on the page. See for example U.S. Pat. No. 5,416,612. Thesetechniques allow the printer to vary the size or optical density of agiven ink dot or pixel, which produces a range of density levels at eachdot or pixel location, thereby improving the image quality. Thusprinting methods that require multiple drop sizes usually depend uponthe way the drops are generated by the print head. As noted above someprint heads have multiple size nozzle diameters, others have circuitryin which the individual ink chambers acccept changing electrical signalsto instruct each chamber how much ink to eject. Still other print headshave nozzles that eject a variable number of small, fixed size dropletsthat are intended to merge then land in a given image pixel location.Printing methods that deposit more than one drop in a pixel location aretypically carried out by multiple printing passes wherein the print headprints a row of pixels pixels multiple times, the image data to theprint head changing in accordance with each pass so that the correctnumber of total droplets deposited at any pixel location is commensuratewith the density required by the processed image data.

The exact relationship between drop size and dot size depends on manyfactors. However, in many cases it can be approximated by the equation

d=a* _(v) ^(b),  (Equ 1)

where d is the diameter of the dot, v is the volume of the drop, a is apositive constant whose magnitude depends on the units of d and v, and bis a positive constant in the range 0.0 to 1.0. This means that theratio of dot size to volume is given by

d/v=a* _(v) ^((b−1)).  (Equ 2)

Therefore, as drop volume goes up the ratio of dot size to drop volumegoes down, which generally means that increasing drop volume providesdiminishing returns in terms of dot size.

Note, however, to achieve full coverage with a multitone ink jet printerit is still necessary that the largest dot have at least a diameter ofp*sqrt(2), and that this largest drop be deposited at each addressablelocation on the raster.

The time required for an ink jet print to dry can be directly related tothe volume of ink deposited on the media. The maximum volume of ink isdetermined by the dot size required to achieve full coverage. In thecase of a binary or multitone printer writing on a raster the dot sizeper pixel required to achieve full coverage has already been shown inFIG. 3 to be one dot with diameter p*sqrt(2).

In the field of inkjet printing it is also well known that if ink dropsplaced at neighboring locations on the page are printed at the sametime, then the ink drops tend to flow together on the surface of thepage before they soak into the page. This can give the reproduced imagean undesirable grainy or noisy appearance often referred to as“coalescence”. It is known that the amount of coalescence present in theprinted image is related to the amount of time that elapses betweenprinting adjacent dots. As the time delay between printing adjacent dotsincreases, the amount of coalescence decreases, thereby improving theimage quality. There are many techniques present in the prior art thatdescribe methods of increasing the time delay between printing adjacentdots using methods referred to as “interlacing”, “print masking”, or“multipass printing”. There are also techniques present in the prior artfor reducing one-dimensional periodic artifacts or “bands.” This isachieved by advancing the paper by an increment less than the printheadwidth, so that successive passes or swaths of the printhead overlap. Thetechniques of print masking and swath overlapping are typicallycombined. See, for example, U.S. Pat. No. 4,967,203 and 5,992,962. Theterm “print masking” generically means printing subsets of the imagepixels in multiple passes of the printhead relative to a receivermedium.

There is a need for improvement over the prior art in ink jet printingto achieve full coverage with a minimum amount of ink so as to minimizethe dry time required for an ink jet print. The prior art utilized largedots with excessive amounts of overlap in order to achieve fullcoverage. This invention provides a method for achieving full coveragewith less dot overlap and with smaller drops, thereby achieving fasterdry times.

SUMMARY OF THE INVENTION

An object of the present invention is to achieve full coverage with lesstotal ink volume, thereby minimizing dry time. This invention relies onmultitone printing capability in combination with a printer that canprint on the shifted raster. As used herein a “shifted raster” implies asubsidiary grid of printing locations that provides dot or pixellocations that are not on the primary or reference raster and whereinspacing between pixel locations on the shifted raster and the referenceraster pixel locations are always less than the nominal spacing betweencenters of recording elements on the printhead.

This invention provides a method of printing and a printer apparatus forreducing the dry time required for an inkjet print by reducing theamount of ink required to achieve full coverage. The method isimplemented in the controller which prepares data for ink jet printingand in the controller which controls the positioning of the ink jet headand the position of the ink receiver, or media.

The invention and its objects and advantages are achieved in accordancewith a first aspect of the invention by an ink jet printer, comprisingan ink jet print head having an array of nozzles, each nozzle beingcapable of selectively producing when actuated at least two ink dropsizes including a larger ink drop size and a smaller ink drop size; anda controller providing, in response to each pixel density signal ofmaximum density value at a respective pixel location, a signal to anozzle to print an ink drop of the larger ink drop size at therespective pixel location on a reference raster and a signal to a sameor different nozzle to print an ink drop of the smaller ink drop size ata pixel location adjacent to the respective pixel location but on ashifted raster.

In accordance with a second aspect of the invention there is provided anink jet printer, comprising an ink jet print head having an array ofnozzles, each nozzle being capable of selectively producing whenactuated an ink drop; and a controller responsive to a pixel densitysignal for controlling the print head for printing, in response to asignal calling for a maximum density value at a respective pixellocation, a larger ink dot at the respective pixel location on areference raster and a smaller ink dot at a pixel location adjacent tothe respective pixel location but on a shifted raster and wherein thelarger and smaller ink dots are of respective sizes such that for a 2×2set or cluster of adjacent ink dots, each of the larger ink dot size andeach at adjacent pixel locations of the reference raster, a gap is leftin the center of the cluster, and the smaller ink dot on the shiftedraster when in the center of the 2×2 set of adjacent larger ink dots isof a size to cover the gap.

In accordance with a third aspect of the invention there is provided anink jet printer, comprising:

a) an ink jet print head having an array of nozzles arranged in at leastone row, each nozzle being capable when actuated of selectivelyproducing a larger drop or a smaller drop at a pixel location on areceiver medium;

b) an ink jet print head drive for moving the ink jet print head in afast scan direction perpendicular to the row of nozzles;

c) a print media driver for moving the receiver medium past the printhead in a slow scan direction orthogonal to the fast scan direction;

d) a controller for controlling the size of drops from the nozzles, theprint head drive and the print media drive for depositing ink drops in areference raster, whereby a 2×2 cluster of four adjacent large ink dropsin the reference raster leaves a gap in the center of the cluster, andfor depositing a smaller ink drop in a shifted raster in response to aprint signal for printing a maximum density dot on the reference rasterwhereby the smaller ink drop covers the gap in the cluster.

In accordance with a fourth aspect of the invention there is provided amethod of printing with an ink jet printer, comprising the steps ofproviding an ink jet print head having an array of nozzles, each nozzlebeing capable of selectively producing when actuated at least two inkdrop sizes including a larger ink drop size and a smaller ink drop size;and in response to each pixel density signal of maximum density value ata respective pixel location on a receiver printing an ink drop of thelarger ink drop size at the respective pixel location on a referenceraster and printing an ink drop of the smaller ink drop size at a pixellocation adjacent to the respective pixel location but on a shiftedraster.

In accordance with a fifth aspect of the invention there is provided amethod of printing with an ink jet printer, comprising the steps ofproviding an ink jet print head having an array of nozzles, each nozzlebeing capable of selectively producing when actuated an ink drop; andprinting, in response to a signal calling for a maximum density value ata respective pixel location, a larger ink dot at the respective pixellocation on a reference raster and a smaller ink dot at a pixel locationadjacent to the respective pixel location but on a shifted raster andwherein the larger and smaller ink dots are of a respective size suchthat for a 2×2 set or cluster of adjacent ink dots each of the largerink dot size and each on the adjacent pixel locations of the referenceraster a gap is left in the center of the cluster, and the smaller inkdot on the shifted raster in the center of the 2×2 set of adjacentlarger ink dots is of a size to cover the gap.

The invention is particularly suited for an ink jet printer that has amaximum dot size capability wherein a square of four pixels printed bythe printer will not provide ink coverage for the center between thefour pixels.

It will be shown that the combination of a drop with dot diameter lessthan p*sqrt(2) on the reference raster and a second drop with smallerdiameter on the shifted raster can achieve full coverage with less totalink volume than either: a) a single drop with dot diameter p*sqrt(2) onthe reference raster, or b) two drops each with dot diameter p, one eachon the reference and shifted rasters.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter of the present invention, itis believed the invention will be better understood from the followingdetailed description when taken in conjunction with the accompanyingdrawings wherein:

FIG. 1A is a schematical perspective view of a printer incorporating theinvention;

FIG. 1B is a perspective view of an ink jet print head module used asone of the print heads in the printer of FIG. 1A;

FIG. 1C is a view of the nozzle plates with nozzle openings for theprint head module of FIG. 1B;

FIG. 2 is a schematic of the nozzle plate shown in FIG. 1C andillustrating an example of a staggered array of nozzle openings;

FIG. 3 is an example of a dot size required for full coverage when a 2×2cluster of four dots are printed;

FIG. 4 is an illustration of a reference raster grid and a shiftedraster grid;

FIG. 5A is an illustration of an alternative nozzle plate which may beused using non-staggered nozzle openings and illustrating plural rows ofnozzles, it being understood that a further nozzle design which may beused comprises only a single row of nozzles;

FIG. 5B is a schematic of a control system for the printer of FIG. 1;

FIG. 6 is an illustration of printing on a reference raster and ashifted raster to demonstrate the concept of the supplementary dot onthe shifted raster and providing for full coverage of ink without anywhite spots showing from the background even though the maximum dot sizeused is advantageously less than that which can provide full coverage toreduce ink consumption and improve drying time;

FIG. 7 is a block diagram schematic view of an image processingarchitecture that may be used with the invention;

FIG. 8 is a schematic of lookup tables which are used to reduce theamount of memory required in the image processing architecture;

FIG. 9 is a schematic of one example of an image chain architecturewhich can be used using various table inputs and outputs;

FIGS. 10(a)-(e) are examples of a set of table values that would beprovided from the main lookup tables selector of FIG. 8 in response to ajob request

FIG. 11 is an illustration of an input request from a RIP requestingvarious dot densities at selected pixel locations and how those requestsare fulfilled during each pass through deposits of ink drops of selecteddot volumes by the printer;

FIGS. 12(a)-(e) and FIGS. 13(a) and (e) are additional examples of setsof table values that are provided by the main lookup table selector inaccordance with different respective job parameter requests,

FIG. 14 is a diagram showing 4 larger drops on a reference raster and 4smaller drops on a shifted raster to achieve full coverage; and

FIG. 15 is a diagram plotting the total ink volume as a function of thelargest dot diameter to achieve full coverage.

DETAILED DESCRIPTION OF THE INVENTION

The present description will be directed in particular to elementsforming part of, or cooperating more directly with, apparatus inaccordance with the present invention. It is to be understood thatelements not specifically shown or described may take various forms wellknown to those skilled in the art.

With reference to FIG. 1A shows an embodiment of printer 10 whichincorporates the invention herein. Reference 11 designates a carriage.An inkjet print head 31 faces the recording element and is mounted onthe print head module 25 (FIG. 11B) which in turn is mounted on thecarriage 11. The carriage 11, coupled through a timing belt 13 with adriver motor (not show), is reproducibly movable longer width of therecording element 12 (in the directions of the arrows A-B), while beingguided by a guide member 15. The inkjet print head 31 receives ink fromthe ink tank 16 through ink supply tube 17. A transport roller 18, whendriven by drive motor (not show), transports the recording medium 12 inthe direction (arrow C) perpendicular to the moving direction of thecarriage 11.

A raster image processor controls image manipulation and the resultantimage file is delivered to the printer via a remotely located computerthrough a communications port. On board memory stores the image filewhile the printer is an operation.

FIGS. 1B and 1C show an embodiment of a piezoelectric print headassembly 25. Reference number 36 designates a nozzle plate having nozzleopenings 37 formed therein. An ink supply port 38 through which inkflows from the ink tank 16 be an ink supply tube 17. The firing rate ofthe print head 31 may be switched between 7.5 kHz and 15 kHz dependingon the selection of image resolution and print quality. The carriagevelocity is fixed in the printer described in all print modes, althoughsuch is not required in accordance with the invention. Althoughillustration is provided of a piezoelectric printhead the invention maybe carried out with other printheads such as thermal and continuousinkjet printheads.

With reference to FIG. 2 the print head 31 for each color of ink to beprinted includes in this embodiment two printhead segments 39A and 39B.Each printhead segment includes two staggered rows of nozzles and eachrow of nozzles has a spacing of {fraction (1/150)} of an inch betweenadjacent nozzles in the row. However, due to the presence of staggeringthere is a nominal nozzle spacing on each printhead segment of {fraction(1/300)} of an inch as indicated in the figure. The nozzles on thesecond segment are similar to that on the first segment and the segmentsare arranged to continue the nominal nozzle spacing for the printhead of{fraction (1/300)} of an inch spacing between nozzles, this nominalnozzle spacing may be generally referred to as “p” when discussingraster grid spacings below. It will be understood that for a printer 10having six different color inks, there will be six similar printheadssimilar to that described for print head 31. The six different colorprint heads arranged on the carriage 11 and as the carriage is traversedacross the receiver sheet 12 for a print pass the nozzles in each of thesix color print heads are actuated to print with ink in their respectivecolors in accordance with image instructions received from the RIP andas such instructions are modified in accordance with the teachingsherein. Typically in printers of this type the number of nozzlesprovided is insufficient to print in entire image during a print passand thus plural print passes are required to print an image with thereceiver sheet being indexed in the direction of the arrow C after eachpass. Thus, it may be said that the images are printed a swath at atime. However, a modification to this last statement exists for thesituation wherein a printing technique known as “print masking” is usedwhich will be explained below. Where print masking is used typically noindexing of the receiver sheet is done until the image that is to beprinted in the swath is printed through multiple passes of the printheadfor the reasons to be described below. In the following description itwill be understood that a print pass may be accomplished also during areturn movement of the nozzles to their starting positions. Anotherfactor which will be introduced in the description herein for theprinting of images a swath at a time is that of printing on the “shiftedraster.”

Thus, the ink jet printer configurations employed herein comprise an inkjet print head that have an array of nozzles. Each nozzle can ejectdrops independently, and each nozzle can eject at least three differentvolumes of ink including a drop of zero volume where essentiallybackground is printed. The print head may be a drop on demand orcontinuous ink jet printing device. An ink jet print head drivemechanism moves the print head in a direction generally perpendicular tothe array of nozzles. This direction is referred to as the fast scandirection. Mechanisms for moving the print head in this direction arewell known and usually comprise providing the support of the print heador carriage on rails, which may include a rail that has a screw thread,and advancing the print head along the rails, such as by rotating therail with the screw thread or otherwise advancing the print head alongthe rails such as by using a timing belt and carriage. Such mechanismstypically provide a back and forth movement to the printhead.Information to the printhead, including data and control signals, can bedelivered through a flexible band of wires or electro-optical link. Asthe print head is transported in the fast scan direction, the nozzlesselectively eject drops at intervals in accordance with enabling signalsfrom a controller that is responsive to image data input into theprinter. The intervals in combination with the nozzle spacing representan addressable rectilinear grid, a raster, on which drops are placed. Apass of the head during which drops are ejected is known as a printpass. The drops ejected during a print pass land on an ink jet medium.After one or more print passes, a print media drive moves the ink jetprint medium; i.e. a receiver sheet such as paper, coated paper orplastic or a plate from which prints can be made, past the print head ina slow scan direction orthogonal or transverse to the fast scandirection. After the print medium or receiver member has been advanced,the print head executes another set of one or more print passes.Printing during a next pass may be while the print head is moving in thereverse direction to that moved during the prior pass. The receivermember may a discrete sheet driven by a roller or other known drivingdevice or the receiver sheet may be a continuous sheet driven, typicallyintermittently, by a drive to a take-up roller or to a feed rollerdrive.

Printheads are also known with one or two parallel rows of nozzles thatare not staggered thus allowing printing of at least certain pixelsusing drops output by two nozzles in succession (see in this regard toFIG. 5A).

Before a print pass, the print medium is lined up with the nozzle arraysuch that the nozzles will eject drops during a print pass onto theraster known as the reference raster. During a subsequent print pass theprint medium may be aligned with the nozzle array such that the nozzleswill eject drops during a print pass onto the raster known as theshifted raster, in which the alignment is adjusted so that the shiftedraster is shifted by one half pixel in the slow scan direction, thisdistance being one-half of the nominal spacing between nozzles on theprinthead. It will be understood that while only a few nozzles areillustrated with a nominal nozzle spacing between nozzle centers of pthat hundreds and even thousands of nozzles may be on a print head ofcertain nominal nozzle spacing of for example {fraction (1/300)}^(th) ofan inch or {fraction (1/600)}^(th) of an inch between nozzle centers.During the print pass for the shifted raster the timing of the intervalsis adjusted so that the shifted raster is also shifted by one half pixelin the fast scan direction from that of the reference raster.

A typical ink jet printer reproduces an image by ejecting small drops ofink from a print head containing an array of spaced apart nozzles, orthe ink drops land on a receiver medium (typically paper) to form roundink dots. In some printers, all drops are the same size, and therefore,all dots are the same size. Normally, these drops are deposited withtheir respective dot centers on a rectilinear grid, a raster, with equalspacing,p, in the horizontal and vertical directions (see FIG. 3).Therefore, to achieve full coverage of the ink it is necessary for thedots, 10, to have at least diameter p*sqrt(2).

Modem ink jet printers may also possess the ability to vary (over somerange) the amount of ink that is deposited at a given location on thepage. Ink jet printers with this capability are referred to as“multitone” or gray scale or “multidrop capable” inkjet printers becausethey can produce multiple density tones at each pixel location on thepage. Some multitone ink jet printers achieve this by varying the volumeof the ink drop produced by the nozzle by changing the electricalsignals sent to the nozzle by varying the diameter of the nozzle. Seefor example U.S. Pat. No. 4,746,935. Other multitone ink jet printersproduce a variable number of smaller, fixed size droplets that areejected by the nozzle (or by plural nozzles during different passes ofthe nozzle array), all of which are intended to merge and land at thesame pixel location on the page.

See for example U.S. Pat. No. 5,416,612. These techniques allow theprinted to vary the size or optical density of a given ink dot, whichproduces a range of density levels at each dot location, therebyimproving the image quality. Thus printing methods that require multipledrops sizes usually depend upon the way the drops are generated by theprint head. As noted above some printheads have multiple size nozzlediameters, others have circuitry in which the individual ink chambersaccept changing electrical signals to instruct each chamber how much inkto eject. Still other printheads have nozzles that ejecting variablenumber of small, fixed size droplets that are intended to merge thenland in a given image pixel location. Printing methods that deposit morethan one drop in the pixel location are typically carried out bymultiple printing passes wherein the printhead prints a row of pixelsmultiple times, the image data to the printhead changing in accordancewith each pass so that the correct number of total droplets deposited atany pixel location is commensurate with the density required by theprocessed image data.

The exact relationship between drop size and dot size depends on manyfactors. However, as drop volume goes up the ratio of dot size to dropvolume goes down, which generally means that increasing drop volumeprovides diminishing returns in terms of dot size.

However, to achieve fill coverage with a multitone ink jet printer, itis still necessary that the largest dot have at least diameter ofp*sqrt2 as illustrated in FIG. 3, and that this largest drop thedeposited at each addressable location on the raster.

The time required for an ink jet print to dry can be directly related tothe volume of ink deposited on the media. The maximum volume of ink isdetermined by the dot size required to achieve full coverage. In thecase of a binary or multitone printer writing on a raster the dot sizeper pixel required to achieve fall coverage has been shown in FIG. 3 tobe one dot with diameter p*sqrt2.

In the field of ink jet printing, it is also well known that if inkdrops placed at neighboring locations on the page are printed at thesame time, then ink drops tend to flow together on the surface of thepage before they soak into the page. This can give the reproduced imagean undesirable grainy or noisy appearance often referred to as“coalescence”. It is known that the amount of coalescence present in theprinted image is related to the amount of time that elapses betweenprinting adjacent dots. As the time delay between printing adjacent dotsincreases, the amount of coalescence decreases, thereby improving theimage quality. There are many techniques present in the prior art thatdescribed methods of increasing the time delay between printing adjacentdots using methods referred to as “interlacing”, “print masking”, or“multipass printing”. There are also techniques present in the prior artfor reducing one-dimensional periodic artifacts or “bands”. This isachieved by advancing the paper by an increment less than the printheadwidth, so that successive passes or swaths of the printhead overlap. Thetechnique of print masking and swath overlapping are typically combined.See, for example, U.S. Pat. Nos. 4,967,203 and 5,992,962. The term“print masking” generally means printing subsets of the image pixels andmultiple passes of the printhead relative to a receiver medium as willbe described below.

As will be noted below, an important feature of the invention relies onmultitone printing capability with printing on the shifted raster.Although the invention is not limited to printers that can print on theshifted raster, there are various advantages that are obtained withprinters with such capability. The use of printing on the shifted rasteras well as the reference raster can provide full coverage using smallersize maximum drops and thereby enhance dry time of the printed dotsreducing coalescence and saving ink. An additional advantages as will beshown herein and as also described in the cross-referenced applicationto Newkirk et al. is that greater multitone levels of printing can beobtained from a printer capable of printing only a relatively few numberof different ink drop volumes.

With reference now to FIG. 4, both a reference raster (shown in darkgrid) and a shifted raster (shown in lighter grid) are identified. Theshifted raster may be shifted by p/2 in both the horizontal and verticaldirections from each of the pixel locations in the primary raster. Asshown in FIG. 4, the reference raster, 20, has spacing p, and theshifted raster, 30, is shifted by p2 in both directions. In response toan image pixel value, the printer may deposit a drop on a receiversheet, the drop being deposited on one or both of the reference rasterand shifted raster. It will be understood that rasters are not printedon the receiver sheet but represent a grid pattern of potential pixellocations.

Although the nozzle pitch dimension described herein is the same as thatof the reference raster grid pitch dimension; i.e. spacing betweencenters of adjacent pixels on the reference raster, the nozzle pitch maynot be identical and the nominal spacing could be greater than thespacing between the reference raster grid lines and accommodation madein the printing mode through control of signals to the printhead in thefast scan direction with printing at appropriate predetermined intervalsto provide a desired pitch spacing for the grid in the fast scandirection and with control of movement of the media in the slow scandirection to provide the desired pitch spacing of the grid in the slowscan direction. It will also be understood that the reference rastergrid need not have the pitch spacing in the fast scan direction that isthe same as that in the slow scan direction. Similarly, the shiftedraster may have similar characteristics as described above for thereference raster. It is preferred to have the pitch spacing betweenpixel centers on the shifted raster be the same as that on the referenceraster with an offset of one-half nominal pitch spacing in twodimensions as illustrated.

Referring now to FIG. 5A, an ink jet printer system is shown in which acontroller, 130, controls a printhead, 140, a print head controller anddriver, 150, and a print media controller and driver, 160. Thecontroller 130, which may include one or more microcomputers suitablyprogrammed, provides signals to the printhead controller and driver 160that directs the print head driver to move the print head in the fastscan direction. While the print head is moving in the fast scandirection, the controller directs the print head to eject ink drops ontothe print medium at appropriate pixel locations for the reference rasterwhen pixels on the reference raster are being printed. In a subsequentpass the controller, while the printhead is moving in the fast scandirection, directs the printhead to eject ink drops onto the printmedium at appropriate pixel locations of the shifted raster when pixelson the shifted raster are being printed. During a single pass printingis only made on one of the rasters, reference or shifted, but not both.Suitable signals are provided to the print head from the print headcontroller so as to print the image data at the appropriate pixellocations on the receiver sheet. After a print pass, the controllermedia controller directs the print media drive 170 to move the printmedium in the slow scan direction. Signals output from the print headcontroller are responsive to data signals input thereto from a suitableelectronic data source that provides a data file of an image to beprinted.

To achieve full coverage, the print head controller 150 directs theprint head to eject an arrangement of drops. In the preferredembodiment, this arrangement consists of a large drop ejected onto thereference raster, and a small drop ejected onto the shifted raster.

Shown in FIG. 6 is an arrangement of drops which illustrate one featureof the invention. For the cluster the arrangement is a three by twocluster of large drops (drops 1-6) placed on the reference raster, and asmall drop (drop “a”) placed on the shifted raster. In a preferredembodiment, the large drops are not large enough to achieve fullcoverage and a gap remains in the center of the cluster. However, asingle small drop is large enough to cover the gap. This arrangement ofdrops not only achieves full coverage but also does so with a lowervolume of ink per unit pixel. The position of the small drop “b” is usedto illustrate the position of the shifted raster to the referenceraster.

FIG. 7 shows a simplified image processing sequence for a monochrome inkjet printer that shows more detail then shown in FIG. 6. In response toa job the request a host computer 180 sends digital signals to a rasterimage processor 181 for conversion of the signals to an image signal i.The image signal i is a two dimensional array composed of individualpicture elements, or pixels, having number of rows w and number ofcolumns h. For color printers, a two dimensional array is created foreach color channel, which in turn corresponds to an ink. For colorprinters, the image signal i is the collective set of two dimensionalarrays. The raster image processor may perform standard image processingfunctions such as the sharpening, resizing, color conversion, andmultitoning to produce a multitone image signal.

For a binary print head, the image signal i for a monochrome printer iscapable of printing only one drop size and only one drop per pixel. Thelocation of each pixel is described by (x,y) coordinates, where x is therow and y is the column. Each pixel contains a numeric code value thatcorresponds to the amount of ink to be placed at the corresponding imagepixel. Thus, the range of pixel code values defines the number ofdifferent density levels that can be printed. In the binary example, thecode values are either 0 or 1, indicating that two density levels can beprinted. It is important to note that the present invention may be usedby any type of printer, preferably multitone printers.

Referring again to FIG. 7, the image signal i is converted to aprinthead image signal o by the swath extractor processor and 182 of areference printhead signal The pass table 183 is a two dimensionallook-up table that contains values of a reference printhead signal as afunction of density leveling pass number. The data values contained inthe pass table 183 may be in a variety of different formats such as willbe explained below. For example, the electronic circuitry that activatesthe print head may be designed to accept ink drop volumes in picoliters.Thus, the electronic circuitry that activates the print head wouldconvert the print head image signal o, which would contain desired inkdrop volumes, into electrical signals that instruct the print head toproduce the desired volumes to form dots of the desired size or opticaldensity. It is important to note that the format of the data values inthe pass table 183 is not fundamental to the invention, and theinvention may be applied to create a printer image signal o for anyparticular print head by using the appropriate data values in the passtables 183.

In providing signals to the print head a swath of data is determined bythe swath extractor processor 182. A swath of data is defined as asubset of multitoned image signal i that is required during one motionof the print head across the page. As noted below mask tables areassociated with the processor 182 to reduce coalescence of adjacent inkdrops by employing print masking. The various tables described hereinmay be stored on a disc storage medium in a computer which implementsthe swath extractor processor. Alternatively, the swath extractorprocessor may be implemented in an imbedded computer within the ink jetprinter and the various tables stored in programmable memory within theprinter. One skilled in the art will recognize that their many differenthardware configurations for the swath extractor processor and manydifferent storage options for the various tables described herein may beconstructed and that the present invention may be applied to any of thedifferent configurations.

The following definitions apply within the context of this document.

Term Definition Print Head A collection of nozzles printing one color ofink comprising one or more integrated sub-assemblies. Print Pass A passof the print head during which ink is ejected onto the receiver media.Swath A rectangular region of the receiver media whose width is equal tothe width of the image and whose height is equal to the height of theprint head. Dot Pitch The horizontal or vertical spacing between pixels,which may be for example either 1/300^(th), 1/600^(th), or 1/1200^(th)of an inch. Reference Raster A 2-D grid of addressable locations, eachlocation associated with a pixel, where the distance between grid pointsis given by the dot pitch. Shifted Raster A 2-D grid of addressablelocations, which is shifted with respect to the reference raster by forexample ½ the dot pitch in each direction. The dot pitches of theshifted raster are for example 1/300^(th) and 1/600^(th) of an inch. Ashifted raster may not the required for 1200 dpi printing. LowResolution Raster A 2-D grid of addressable locations which may be asubset of the reference raster. (Printhead Resolution This is determinedby the native resolution of the printhead (300 DPI printhead). Raster)For 300 dpi printing, the reference raster and the low resolution rasterare the Same, Both having dot pitches of 300 dpi. For 600 dpi printing,the reference raster is 600 × 600 dpi and equals the union of twointerleaved low resolution rasters at 300 × 600 dpi. For 1200 dpiprinting the reference raster is 1200 × 1200 dpi and equals the union offour interleaved low resolution rasters at 300 × 1200 dpi. LowResolution Shifted A 2-D grid of addressable locations which may be asubset of the shifted raster Raster corresponding to the low resolutionraster. For 300 dpi printing, the shifted (Printhead Resolution rasterand the low resolution shifted raster are the same, both having dotpitches of Shifted Raster) 300 dpi. For 600 dpi printing, the shiftedraster is 600 × 600 dpi and equals the union of two interleaved lowresolution shifted rasters at 300 × 600 dpi. For 1200 dpi printing, ashifted raster is not used. Low Resolution Accumulated count duringprinting of the number of times the print head has beenAccumulator(A_(LR)) positioned to print on the low resolution rasterwhich combines a reference raster And shifted raster. One of these isrequired during 300 dpi printing, and two are required during 600 dpiprinting. These accumulators are not used during 1200 dpi printing.Reference Raster Accumulated count during printing of the number oftimes the print head has been Accumulator(A_(RR)) positioned to print onthe reference raster. One of these is required during 300 dpi printing,and two are required during 600 dpi printing. These accumulators are notused during 1200 dpi printing. Shifted Raster Accumulator Accumulatedcount during printing of the number of times the print head has been(A_(SR)) positioned to print on the shifted raster. One of these isrequired during 300 dpi printing, and two are required during 600 dpiprinting. These accumulators are Not used during 1200 dpi printing.Resolution Passes (N_(R)) Minimum number of print passes that arerequired to achieve the desired dot Pitch. For 300 dpi printing N_(R) =1, for 600 dpi printing N_(R) = 2, and for 1200 dpi Printing N_(R) = 4.Banding Passes (N_(B)) Extra print passes that are required to isolatethe ink droplets both spatially And temporally. Allowed values forexample are {2,4,8}. So for example a value of 2 implies the drops aredistributed over 2 print passes. Shifted Passes (N_(S)) Extra printpasses that are required to print on the shifted raster. Allowed Valuesare 1 or 2. Total Passes (N_(T)) Total number of print passes requiredto print all drops in a swath, where N_(T) = N_(R) · N_(B) · N_(S.) LowResolution Passes(N_(LR)) Number of print passes required to print alldrops in a swath on one of the Low Resolution rasters and thecorresponding low resolution shifted raster, where N_(LR) = N_(B) ·N_(S.)

Consider the examples in the following table:

Mode Example (dpi, bits/pix, Number banding passes) Ink Volumes (pl)N_(R) N_(B) N_(S) N_(T) _(LR) 1 300/1/2 0, 64 1 2 1 2 2 2 300/1/2 0, 721 2 2 4 4 3 300/2/2 0, 16, 48, 64 1 2 1 2 2 4 300/2/2 0, 16, 48, 72 1 22 4 4 5 300/4/2 0, 8, 16, 32, 48, 64, 72 1 2 2 4 4 6 300/4/2 0, 16, 24,32, 40, 48, 56, 64, 72 1 2 2 4 4 7 300/4/2 0, 8, 16, 24, 32, 40, 48, 56,64, 72 1 2 2 4 4 8 600/2/2 0, 16, 32 2 2 1 4 2 9 600/2/2 0, 8, 16, 24 22 2 8 4 10  1200/1/4  0, 8 4 4 1 16  4

The “Ink Volumes” column lists possible ink volumes (in picoliters)which could be associated with the raster code values in that mode. Inexample 1, a raster image processor (RIP) outputs to the printhead imagedata at a minimal resolution of 300 DPI. The printing resolutiondesignated by the RIP is printing; i.e. one bit per pixel bit depthprinting. In order to reduce coalescence of ink drops where two adjacentink drops are deposited substantially simultaneously, the prior artrecognizes the desirability of employing a technique known as printmasking to employ two or more passes of the printhead across the imagewherein during the first pass each 2×2 grid of pixels may have pixelsarranged along a first diagonal of the grid printed and during thissecond pass pixels arranged along a second diagonal of the grid may beprinted. Print masking logic tables for printing during the first passand the second pass are typically provided. In the logic table passtables described below a “1 ” in this example indicates the pixellocation that may be printed during the pass if the image data sospecifies printing of a dot at that pixel location. A “0” indicates thatduring such pass and at that pixel location no dot may be printed eventhough the data identifies that location for printing of dot. The logictable for the second pass is complementary to that of the first pass sothat data to be printed at respective pixel locations will be printedduring one of the two passes. In the example of print masking techniqueillustrated it will be noted that during a pass available pixelpositions for placement of drops are restricted to being along adiagonal. Other known print masking techniques employ different dropplacement algorithms. For example, in a 2×2 grid of pixels somealgorithms only select one of the four available positions for printingduring a pass and thus four passes are required to print an image swathon a reference raster. Thus, the example number 1 is fully explained.

In considering the next example, example 2, there is an assumption thatthe maximum drop volume output by a nozzle at a pixel location at onetime is 64 picoliters. In example 2 the raster image processor hasoutput image data at a nominal resolution of 300 DPI with one bit perpixel bit depth which as noted for example 1 is printed in two bandingpasses to take advantage of print masking to avoid drop coalescence ofadjacent drops. The number of banding passes will be stated by the RIPas part of its program. As noted above the number of banding passes inthis example may be 1, 2,4 or 8. In comparing example 1 with example 2,it will be noted that although the binary image file coming from the RIPis identical to example 1 the RIP has also identified a requirement forprinting with a drop size of 72 picoliters. The printer in response tothis command from the RIP accommodates the command by printing a 64picoliter drop at the identified pixel location on the reference rasterand a supplementary 8 picoliters drop at an adjacent location on theshifted raster grid. The concept of printing on the shifted raster isillustrated in FIG. 6 wherein pixels 1-6 are maximum size pixels thatcan be printed on the reference raster grid and having the indicatedresolution of {fraction (1/300)}th of an inch. A secondary grid also of{fraction (1/300)} of an inch but separated from the main grid by{fraction (1/600)} of an inch in both the pass direction and mediaadvancement direction (Arrow C) is also provided, it being understoodthat the grid lines are not printed on the media but represent possiblepixel printing locations. In this regard printing by the printheadduring a print pass across the media, as indicated by the arrow in FIG.6 of the pass direction, will be during any pass either for placement ofink drops at pixel locations on the reference raster or the shiftedraster but not both during such pass. Thus this will explain thedifferences between the number of total passes printed for the example 2is 4 while the total number of passes printed for example 1 is 2, therebeing two additional passes in example 2 for printing the supplementarypixels on the shifted raster to accommodate the larger drop size requestby the RIP.

For Examples 3 and 4, it will be noted that the RIP is calling forpixels to be placed at 300 DPI print resolution with a two bits perpixel bit depth and using two banding passes (print masking). In example3 all the ink drop sizes requested by the RIP are within the capabilityof the printhead at that print resolution and no printing on the shiftedraster is required. Thus the total number of passes in example 3 aretwo. However, in example 4 the requirement for printing with a 72picoliters drop size requires printing on the shifted raster toaccommodate the extra drop size and two more total passes are required.

In Examples 5, 6 and 7, the RIP is requesting printing at 300 DPIresolution with four bits per pixel bit depth and using two bandingpasses. However, in all of these examples drop sizes of 72 picolitersare requested. Thus, printing on the shifted raster is required in theexamples 5,6 and 7 and total number of passes are 4 in each example,because the use of print masking requires two banding passes forprinting on the reference raster and two banding passes for printing onthe shifted raster to avoid coalescence of ink drops.

Although in reading above the impression might be obtained that printingon the shifted raster is only for providing supplementary drops forprinting of the drop volume beyond that of the capability of the printernozzle the concept of printing on the shifted raster also contemplatesthat drop sizes not available to the printhead during a particularprinting mode may also be accomplished through printing using theshifted raster. For example, consider that the printhead has the abilityof printing at say a 300 DPI resolution drop volumes of say 0,8,16,32,48and 64 picoliters from each nozzle as the printhead traverses across theprint media. Since the concept of the shifted raster is being used inany event to accommodate requests by the RIP for printing ink drops of72 picoliters other drop sizes requests may also be accommodated as willnow be described. A drop size request for printing a drop volume of 24picoliters may be accommodated by printing a 16 picoliters drop on themain raster and a supplementary 8 picoliters drop on the shifted rasterat adjacent location to the 16 picoliters drop. Other intermediate dropvolumes may be accommodated with the RIP being totally ignorant thatthis is occurring since the elegance of the concept of the shiftedraster has the implementation carried out by the printer while theraster image processor is totally ignorant of the fact that theprinthead is only capable of printing the five drop sizes identifiedabove but yet may be requesting printing of number of drop sizes beyondthat of the normal printer capability. The concept of the shifted rastermay also be extended by printing drop sizes not of just say the samesupplementary 8 picoliters drop sizes on the shifted raster grid. Forexample at a request for printing of a drop size of 80 picoliters, theprinthead may fulfill this request by printing a 64 picoliters drop onthe reference raster and a 16 picoliters drop on the shifted raster.Thus, the concept of the shifted raster extends the exposure spacecapability of the printer or the effective number dot sizes or opticaldensities that may be printed by the printer. A person inputting a printjob from a host computer can designate, for example, that printing bedone with 9 or 10 pixel sizes at say 300 DPI print resolution. Although,the printer nozzles can only produce drop volumes of say five differentsizes the job can still be printed by the printer, with the RIP assumingthat the printer has the ability to print the requested 9 or 10 dropvolume sizes. As will be described below, the printer is adapted toaccommodate these requests by recognizing which pixels need to beaccommodated through the printing of supplementary drops on the shiftedraster. It is important to keep in mind that once printing on theshifted raster is being done, for example to accommodate a request for apixel of 72 picoliters by printing the 8 picoliters supplementary dropon the shifted raster, an 80 picoliters drop volume request may be alsoaccommodated during the same band passes for shifted raster printing aswell as any of the other supplementary drops required for accommodatingintermediate drop sizes that are not available to the printhead whenprinting on the reference raster. Thus, although concept of shiftedraster has increased the number of band passes, the additionalflexibility of providing for greater bit depth printing is an addedbenefit. A further benefit to the concept of printing on the shiftedraster is also provided in that as noted for FIG. 6 full coverage of anarea can be provided without applying excessive drop sizes to accomplishsame and thus faster drying of prints can result. In accordance with theinvention, a supplementary drop deposited between a cluster of say fourpixels on the reference raster in a 2×2 configuration can fill in awhite space where the largest drop size printed does not providesufficient overlap to eliminate the white spot between the cluster offour dots. The description herein clearly shows that for enhanced dryingit is better to rely on smaller drops for printing with use of shiftedraster and to provide a fill in drop of the same color ink for completecoverage rather than to use four large drop sizes that have substantialoverlap.

Examples 8 and 9 in the chart above illustrate the use of the shiftedraster in 600 DPI mode as well as the penalty for placing 8 pl drops onthe Shifted Raster.

Example 10 illustrates printing of a request at 1200 DPI resolutionwherein the bit depth is one or binary. Because of the closeness of thedots placed on the reference raster four banding passes are requested bythe RIP for print masking. No printing on the shifted raster is done inthis example.

It will be understood that after each set of banding passes for printinga swath on say the reference raster, the receiver sheet is indexed asmall distance according to the print mode. Thus, for example, for thefirst seven examples above the printing resolution on the referenceraster is 300 DPI and after the two banding passes for printing on thereference raster the receiver sheet will be indexed {fraction (1/600)}of an inch for printing on the shifted raster if required in that mode.The printhead is also controlled to print across the receiver sheet 300DPI on the shifted raster but the locations for the pixels on theshifted raster grid are shifted {fraction (1/600)} of an inch from thoselocations on the reference raster. Thus for printing at 300 DPIresolution the printhead moves in {fraction (1/300)} increments forprinting whether it is printing drops on the reference raster or theshifted raster.

The shifted raster approach can be extended to 600 DPI. In that mode theraster would be shifted {fraction (1/1200)}th of an inch in eachdirection. The shifted raster approach can be used in combination withbi-directional or uni-directional printing, as well as an arbitrarynumber of banding passes.

The shifted raster mode and the use of 8 pL drops to satisfy the aboveconstraint is preferably directed by look up tables in the printer aswill be described below.

In accordance with the invention an image chain architecture is providedfor the printer which optimizes discrete drop volumes using a variety ofmedia receivers. Six factors determine the selectable imaging chainoperations in the print engine which produce optimized drop volumesdepending on a given combination of factors. Each of these factors arerequested by the RIP for each print job sent to the printer. Thesefactors are:

a) Resolution (DPI);

b) Bit Depth or Bits per pixel (BPP);

c) Number of banding passes (a print masking consideration);

d) Printing direction, printing in forward only (unidirectional) andprinting also during return (bi-directional);

e) Ink;

f) Media

Examples for combinations of DPI, BPP and banding passes that comprise aPrint Mode:

DPI Bits per Pixel Banding Passes 1200 1 2 1200 1 4 600 2 2 600 2 4 3001 2 300 2 4 300 4 2 300 4 4 300 4 8

Since each of these combinations can be printed uni-directionally orbi-directional there are, therefore, a total of 18 print modes which canbe selected with every combination of ink and media in this example. Itwill be understood of course that the above are just examples and arenot provided as limitations of the invention herein.

With reference to FIG. 8, The RIP specifies DPI, ink type and mediatype. This results in a significant number of drop volume possibilitiesfor the printer to deal with for each and every media. The CoverageFactor LUT 210 reduces the number of options available. This LUT willdefine, for every possible combination of DPI, ink types and mediatypes, which factor to use.

Coverage factor is internal to the printer and known only by theprinter. The RIP does not have access to this parameter, thereby greatlyreducing the complexity of the host software programs that interface tothe printer. The coverage factor is determined experimentally by usingall different combinations of DPI, ink and media and making numerousdifferent sample prints. In the example provided there are threepermitted printing resolutions (DPI). Two ink types (this may beoptional for some printers may assume only one type ink is to be used;i.e. dye or pigment types) are also assumed. Twelve different mediatypes are also assumed which represent media of different types ofsurfaces that are suited for use with the printer as a job request. Aswill be shown below, these 72 combinations represent quite a largenumber. However, by scanning of all the various combinations of printsmade a determination can be made of possible overlaps or equivalenciesregarding ink coverage. The 72 combinations can be reduced down to, forexample, six different coverage factors, this number not being criticalit being understood that it comprises a substantial reduction fromhaving the 72 brute force combinations. The coverage factor lookup tablemay be a 3×2×12 table that maps the DPI, ink and media into a coveragefactor.

In response to the input combination of a specific resolution (DPI), inkand media by the RIP, a code representing the coverage factor found forthis combination is output to a main look up table selector to 220. Alsoinput to this look up table are other factors. The RIP will furtherspecify resolution (DPI), bit depth (BPP), number of banding passes, anddirectionality (printing in one or two directions). The printer knowsthe ink and media. The DPI, ink and media determine the coverage factorvia the Coverage Factor LUT.

The DPI, BPP, directionality and banding passes are combined with thecoverage factor by the Main LUT selector to select which tables are tobe used for the imaging chain.

For example, assume dye ink and glossy media are loaded in the printer.The RIP may prepare an image for printing at 300 DPI. This combinationof ink and media at 300 DPI may require 72 pL at each pixel to achievefull coverage for said media, therefore, the coverage factor wouldindicate a print mode which delivers 72 pL at the max code value. Inaddition, the RIP would specify to the printer the BPP, number ofbanding passes, and print direction. This information, combined with theDPI, would identify one of 18 supported print modes. The identifiedprint mode combined with the coverage factor would identify one of 108sets of imaging chain tables. Each set of imaging chain tables includesall the tables, LUTs, and matrices needed to define the imaging chainfor each color. Drop size mapping, print masking and shifted rasterprinting are accomplished using a Pass Table, Print Mask, Shifted RasterLUT and Drop Volume LUT. These operations are applied to the multitoneimage data received from the host, and illustrated logically in FIG. 9.

Pass Tables translate multitone data from the host, into ink volumeindices representing ink drop volumes that are to be printed on themedia. There are two Pass Tables 230, one is used when printing on thereference raster and the other is used when printing on the shiftedraster. A Pass Table has one row for each multitone level and onecolumn. The data entries in the Pass Table are drop volume indices. Inthe example provided there are 5 drop volume sizes that each nozzle isable to provide. The drop volume indices are stored as 3-bit numbers.The drop volume indices are translated through the Drop Volume LUT tospecify the actual volume of ink placed on the page. Two sample PassTables are shown in FIGS. 10(a) and 10(b), where drop volume indices areindicated by letters to reduce confusion. The example of FIGS. 10(a)through 10(e) the RIP is requesting printing at 300 DPI, 2 bots perpixel, bit depth using 2 banding passes for print maskingconsiderations.

These Pass Tables assume the Drop Volume LUT 260 shown in FIG. 10(e). Inthese Pass Tables, a multitone level of zero results in no ink beingplaced on the page. A multitone level of one results in a single 16 pldrop being printed on the reference raster. A multitone level of tworesults in a single 48 pl drop on the reference raster. Lastly, amultitone level of three results in a single 64 pl drop on the referenceraster and an 8 pl drop on the shifted raster.

The Shifted Raster LUT 240 indicates when to print on the shiftedraster. A Shifted Raster LUT is applied to a low resolution raster andits corresponding low resolution shifted raster. Therefore, there is onerow for each print pass in the low resolution raster and thecorresponding low resolution shifted raster, combined. The entries areeither True (T) or False (F), where True means print on the shiftedraster. A sample Shifted Raster LUT is shown in FIG. 10(c).

This Shifted Raster LUT indicates that the printer should print on theshifted raster on all the odd number print passes. Entries are read fromthe Shifted Raster LUT according to

ShiftIndicator=ShiftedRasterLUT[A _(LR)%N _(LR],)

where % is the mod operator, and A_(LR) is the low resolutionaccumulator. As noted before, one accumulator is required for 300 DPIprinting, however, two are required for 600 DPI printing. This isbecause for 600 DPI printing there are two interleaved low resolutionrasters.

It may be decided in the interest of productivity that shifted rasterprinting will always be executed in bi-directional print mode, even ifthe RIP has requested uni-directional printing. Therefore, the printermust recognize that the print mode will require the shifted raster andoverride the RIP request if necessary. The entries in the Shifted RasterLUTs preferably alternate T, F, T, F, etc. Therefore, the referenceraster, with bi-directional printing, will always be printed in onedirection and the shifted raster will be printed in the other direction.

It may be decided to design the imaging chain architecture so that theshifted raster is only used in print modes that are referred to by theRIP as uni-directional, and never in print modes that are referred to bythe RIP as bi-directional. The reason for this is that if auni-directional print mode is defined to use the shifted raster, thenthe corresponding bidirectional print mode to NOT use the shifted rasterwould also be desirably be defined. Otherwise, a user of the printerwill see no productivity difference between the two modes. Independentof this decision, it may be desirable to also define a uni-directionalprint mode to not use the shifted raster and a correspondingbi-directional print mode to NOT use the shifted raster also. Otherwise,again, the user of the printer will see no productivity differencebetween these two modes.

Print masking as noted above distributes the drops spatially andtemporally over the available print passes. The Print Mask table 250, anexample of which is shown in FIG. 10(d), is used to logically split theinput image, on each low resolution raster and each low resolutionshifted raster, up into data buffers (not shown). The following printmask equations may be used to combine the Print Mask and Pass Tables.Equation (1) is selected if the Shifted Raster LUT indicates that thenext print pass shall be on the reference raster, and equation (2) isselected if the Shifted Raster LUT indicates that the next print passshall be on the Shifted Raster.

data[i][j]=RRPassTable[input[i][j]]& ((mask[i%m _(x) ][j%m _(y)])=(A_(RR)%N _(B)))  (1)

data[i][j]=SRPassTable[input[i][j]]& ((mask[i%m _(x) ][j%m _(y)])=(A_(SR)%N _(B)))  (2)

In these equations, input[i][j] is the multitone level at pixel (ij) inthe multitone input image, A_(RR) and A_(SR) are the reference rasterand shifted raster accumulators, mask is the print mask, (m_(x), m_(y))is the width and height of the print mask, RRPassTable is the ReferenceRaster Pass Table, SRPassTable is the Shifted Raster Pass Table, N_(B)is the number of banding passes, data is the image data passed to theremainder of the imaging chain, and the percent symbol, %, indicates themod operation.

According to the equation, the appropriate accumulator is moded by thenumber of banding passes. The result of this operation is compared witha value from the print mask which is tiled across the image in bothdirections. If the compare is true then a value from the appropriatepass table is passed to the data buffer. If the compare is false then azero (no drop) is passed to the data buffer. The values in the printmask are in the range 0 to N_(B)−1.

The Drop Volume LUT provides the translation from drop index to ink dropvolume. The Drop Volume LUT 260 may be fixed and need not be changed.

To illustrate the use of the tables of FIGS. 10(a)-(e), consider withreference to FIG. 11 a 4×6 section of a multitone image received by theprinter from the host. The printing mode is 300 DPI, 2 bits/pixel, 2banding passes and since the shifted raster is used, N_(T)=4. This imageis sent to a printer that is using the Pass Tables, Shifted Raster LUT,Print Mask, and Drop Volume LUT illustrated in FIG. 10.

Shown for each pass is the drop volume of ink in picoliters that shouldbe placed on the page at each raster location. As indicated in thefigure for this example, the pixel in the upper left corner isconsidered the (0,0) location. The 2-bit pixel in that location receivedfrom the host has a value of 0.

On the first print pass the value of A_(LR) is zero, therefore, theShifted Raster LUT indicates that the first print pass should be on theReference Raster. Referring then to print masking equation (1) the valueof A_(RR) should be used. Since this is the first print pass the valueA_(RR) is also zero, therefore the mod with N_(B) is zero. Secondly, thevalue in the Print Mask corresponding to first line and first pixel isalso a zero, therefore, a drop volume index will be selected from theReference Raster Pass Table. Row zero of the Reference Raster Pass Tableis selected because the pixel value is 0. Therefore, it is seen thatthis pixel is rendered on the first pass of the reference raster with adrop of index A. From the Drop Volume LUT in section 0 this correspondsto a 0 pl drop.

The next pixel in the line, i.e., at location (0,1), has a value of 1.The value of A_(LR) and A_(RR) are still zero since no print passes havebeen completed. However, the value in the Print Mask corresponding tothis line and this pixel location is a one. Since this is not zero, nodrop volume index will be selected from the Reference Raster Pass Table.Therefore, it is seen that this pixel is also rendered on the first passof the reference raster with a drop of index A. From the Drop Volume LUTthis corresponds to a 0 pl drop.

The next pixel in line at location (0,2) also has a value of 1. Sincethe Print Mask is tiled across the image in both directions, the valuein the Print Mask corresponding to this line and this pixel location isa zero. Therefore, the drop volume index will be selected from theReference Raster Pass Table. Row one of the Reference Raster Pass Tableis selected because the pixel value is 1. Therefore, it is seen thatthis pixel is rendered on the first pass of the reference raster with adrop of index C. From the Drop Volume LUT this corresponds to a 16 pldrop.

The rest of the pass buffer is constructed in the same fashion and thedrops deposited on the page. After the first print pass the value ofA_(LR) and A_(RR) are incremented. For the next print pass the value ofA_(LR) is one, therefore, the Shifted Raster LUT indicates that the nextprint pass should be on the Shifted Raster. The paper should bepositioned so that the nozzles are lined up with the shifted raster.

Considering again the pixel in the upper left corner, at location (0,0),the 2-bit pixel in that location received from the host has a value of0. Referring then to print masking equation (2) the value of A_(SR)should be used. Since this is the first print pass over the shiftedraster the value A_(SR) is zero. Secondly, the value in the Print Maskcorresponding to first line and first pixel location is also zero.Therefore, the drop volume index will be selected from the ShiftedRaster Pass Table. Row zero of the Shifted Raster Pass Table is selectedbecause the pixel value is 0. Therefore, it is seen that this pixel isrendered on the first pass of the shifted raster with a drop of index A.From the Drop Volume LUT this corresponds to a 0 pl drop.

Continuing to fill the pass buffer for a print pass over the shiftedraster, consider the last pixel in the second line, at location (1,5),which has a value of 3. Since the Print Mask is tiled across the imagein both directions, the value in the Print Mask corresponding to thisline and this pixel location is a zero. Since the value of A_(SR) isstill zero, the drop volume index will be selected from the ShiftedRaster Pass Table. Row three of the Shifted Raster Pass Table isselected because the pixel value is 3. Therefore, it is seen that thispixel is rendered on the first pass of the shifted raster with a drop ofindex B. From the Drop Volume LUT this corresponds to an 8 pl drop.

The rest of the pass buffer is constructed in the same fashion and thedrops deposited on the page on the shifted raster. After the print passthe value of A_(LR) and A_(SR) are incremented. For the next print passthe value of A_(LR) is two, therefore, the Shifted Raster LUT indicatesthat the next print pass should be on the reference raster and the papershould be positioned so that the nozzles are lined up with the referenceraster.

With reference to FIGS. 12(a)-(e), there is shown an example of theprint mode wherein the RIP is requesting printing at 300 DPI resolutionat 4 bits per pixel bit depth and using four banding passes for printmasking considerations. The 4 bits per pixel from the RIP are requestingeleven ink volumes shown (zero, 8, 16, 24, 32, 40, 48, 56, 64, 72, 80).However, as noted from the Drop Volume LUT each nozzle is only adaptedto print six drop volume sizes including zero. However, using shiftedraster a 24 picoliters drop volume may be created using a drop of 16picoliters on the reference raster and a drop of 8 picoliters on theshifted raster. Similarly, a 24 picoliters drop may be simulated using a16 picoliters drop on the reference raster and an 8 picoliters drop onthe reference raster. This is also true for a 40 picoliters drop whichis simulated by a 32 picoliters drop on the reference raster and an 8picoliters drop on the shifted raster and for a 56 picoliters drop whichis simulated by a 48 picoliters drop on the reference raster and an 8picoliters drop on the shifted raster. The production of the 72picoliters drop using a 64 picoliters drop on the reference raster andan 8 picoliters drop on the shifted raster has been discussedpreviously. As also noted above the shifted raster may also be used into produce larger size drops then the 8 picoliters noted in the previousexamples. Thus an 80 picoliters drop is produced during printing on theshifted raster by printing a 16 picoliters drop on the shifted rasterwith the 64 picoliters drop on the reference raster pixel locationadjacent thereto.

With reference to FIGS. 13(a)-(e), there is shown still another exampleof a print mode. In this example the RIP is requesting printing at 300DPI resolution, 4 bits per pixel bit depth, and two banding passes forprint masking considerations. For such printing, assume that only sevendot sizes are to be used for printing (0, 8, 16, 32, 48, 64, 72). Inthis example, all the dot sizes may be accommodated by the print headexcept for 72 for which the shifted raster is employed as will now beobvious from the above description.

It will be noted in the various examples presented herein that during aset of passes involving consecutive motions of the print head over thesame region of the page for example for printing on the reference rasterthat the receiver has not moved until all the passes are completed.However, interlacing techniques are known or the page may be advancedfor example ¼th of the print head height after each pass. The inventionapplies equally well to such forms of print masking or interlaced swathprinting.

The arrangement of drops not only achieves full coverage but also doesso with a lower volume of ink per unit pixel. Consider a typical dotdiameter to drop volume relationship given by

d=15*v ^(0.5),  (Equ3)

where d is the dot diameter in microns and v is the drop volume inpicoliters. Therefore, the drop volume is given by

v=d ²/225.  (Equ 4)

The total drop volume per unit pixel to achieve full coverage is givenby the sum of both the large drop volume and the small drop volume,i.e.,

V _(t) =V ₁ +V ₂,  (Equ 5)

where V₁ is the volume of the large drop and V₂ is the volume of thesmall drop. At the one extreme the large drop is large enough to achievefull coverage as shown in FIG. 3, and therefore V₂=0. At the otherextreme the large drop and the small drop are the same size (not shown)and therefore V_(1=V) ₂. It can be shown that given the raster spacing,p, and the diameter of the large drop, d₁, then in order to achieve fullcoverage the diameter of the small drop, d_(2,) must be at minimum givenby

d ₂=(p−(d ₁ ² −p ²)^(0.5)).  (Equ 6)

Combining equations (4), (5) and (6) yields

V _(t)=(2/225)(d ₁ ² −p(d ₁ ² −p ²)^(0.5))  (Equ 7)

This equation is plotted in FIG. 15 for p=85 microns which approximatesthe raster spacing for 300 DPI. At the one extreme where a small drop isnot needed, the dot diameter corresponding to the large drop is given by85*2^(0.5)=120 microns. At the other extreme in which the large drop andsmall drop are the same size, both diameters are equal to the rasterspacing, i.e., 85 microns. It can be seen from FIG. 15 that for everydrop size between these two extremes less total ink volume is requiredto achieve full coverage.

One skilled in the art will recognize that relationship plotted in FIG.15 is based upon the typical dot size to drop volume relationship statedin equation 3. The shape of the curve in FIG. 15 will vary depending onthe particular dot size to drop volume relationship chosen. Ink jet inksused to image recording elements used in the present invention are wellknown in the art.

Other implementations of the invention may rely upon the collection orexamination of image data to determine if for example 2×2 clusters arepresent in the image data to be printed on the reference raster. Whereclusters are found these are indicative that full coverage is requiredand a supplementary drop on the shifted raster may be provided for. Thuspresence of a cluster may be used as a criterion selection fordetermining when a supplementary drop is to be deposited on the shiftedraster to provide for full coverage with reduced consumption of ink.

Image recording media used in the present invention are well known inthe art. Examples of recording media include, but are not limited to,bond paper, sized papers, vinyls, textiles, matte coated papers, andphoto quality papers having satin, semi-glossy, or glossy finishes.

The various implementations shown here are exemplary and as noted abovemay be practiced in different forms using a computer or discretecomponents for performing the logic operations described.

This invention describes a printing method and apparatus of achievingfull coverage on a receiver medium using an inkjet printer employingmultitone printing and a shifted raster. The invention thus provides forfull ink coverage in interstitial pixel locations which would otherwisebe left blank or which might otherwise be filled but with inefficientuse of ink. The invention thus provides for improvements in dry time ofthe ink when printing with full coverage.

The invention has been described with particular reference to itspreferred embodiments, but it will be understood by those skilled in theart that various changes may be made and equivalents may be substitutedfor elements of the preferred embodiments without departing from theinvention. In addition, many modifications may be made to adapt aparticular situation and material to a teaching of the present inventionwithout the departing from the essential teachings of the invention.

What is claimed is:
 1. An ink jet printer, comprising: an ink jet printhead having an array of nozzles, each nozzle being capable ofselectively producing when actuated at least two ink drop sizesincluding a larger ink drop size and a smaller ink drop size; and acontroller providing, in response to each pixel density signal ofmaximum density value at a respective pixel location, a signal to anozzle to print an ink drop of the larger ink drop size at therespective pixel location on a reference raster and a signal to a sameor different nozzle to print an ink drop of the smaller ink drop size ata pixel location adjacent to the respective pixel location but on ashifted raster.
 2. The printer of claim 1 wherein the array of nozzlesare arranged in at least one row and including a print head drive formoving the ink jet print head in a fast scan direction perpendicular tothe row of the array of nozzles; and print media drive for moving an inkjet print medium past the print head in a slow scan direction orthogonalto the fast scan direction.
 3. The inkjet printer of claim 2 and whereinink drops are of a size such that when depositing a 2×2 set or clusterof adjacent ink drops of the larger ink drop size in a reference rastera gap is left in the center of the cluster, and an ink drop of thesmaller ink drop size when deposited on the shifted raster in the centerof the 2×2 set of adjacent ink drops covers the gap.
 4. An ink jetprinter, comprising: an ink jet print head having an array of nozzles,each nozzle being capable of selectively producing when actuated an inkdrop; and a controller responsive to a pixel density signal forcontrolling the print head for printing, in response to a signal callingfor a maximum density value at a respective pixel location, a larger inkdot at the respective pixel location on a reference raster and a smallerink dot at a pixel location adjacent to the respective pixel locationbut on a shifted raster and wherein the larger and smaller ink dots areof respective sizes such that for a 2×2 set or cluster of adjacent inkdots, each of the larger ink dot size and each at adjacent pixellocations of the reference raster, a gap is left in the center of thecluster, and the smaller ink dot on the shifted raster when in thecenter of the 2×2 set of adjacent larger ink dots is of a size to coverthe gap.
 5. The printer of claim 4 and wherein the controller controlsthe print head so that the larger dot on the reference raster is formedby multiple passes of the print head wherein an ink drop is deposited atthe pixel location on each of the plural passes.
 6. The method of claim4 and wherein the controller controls the print head so that the largerdot on the reference raster is formed in only a single pass of the printhead wherein a larger ink drop is deposited at the pixel location duringthe single pass during which pass only dots on the reference raster areprinted and the smaller dot is formed by a smaller ink drop depositedduring only another single pass of the print head during which anothersingle pass of the print head printing of dots only on the the shiftedraster occurs.
 7. An ink jet printer, comprising: a) an ink jet printhead having an array of nozzles arranged in at least one row, eachnozzle being capable when actuated of selectively producing a largerdrop or a smaller drop at a pixel location on a receiver medium; b) anink jet print head drive for moving the ink jet print head in a fastscan direction perpendicular to the row of nozzles; c) a print mediadrive for moving the receiver medium past the print head in a slow scandirection orthogonal to the fast scan direction; d) a controller forcontrolling the size of drops from the nozzles, the print head drive andthe print media drive for depositing ink drops in a reference raster,whereby a 2×2 cluster of four adjacent large ink drops in the referenceraster leaves a gap in the center of the cluster, and for depositing asmaller ink drop in a shifted raster in response to a print signal forprinting a maximum density dot on the reference raster whereby thesmaller ink drop covers the gap in the cluster.
 8. The ink jet printerclaimed in claim 7, wherein the reference raster is formed by set ofmultiple passes of the print head and only dots on the reference rasterare printed during each of the multiple passes of the set.
 9. The inkjet printer claimed in claim 8, wherein the shifted raster is formed bya second set of multiple passes of the print head and only dots on theshifted raster are printed during each of the multiple passes of thesecond set.
 10. A method of printing with an ink jet printer, comprisingthe steps of: providing an ink jet print head having an array ofnozzles, each nozzle being capable of selectively producing whenactuated at least two ink drop sizes including a larger ink drop sizeand a smaller ink drop size; and in response to each pixel densitysignal of maximum density value at a respective pixel location on areceiver printing an ink drop of the larger ink drop size at therespective pixel location on a reference raster and printing an ink dropof the smaller ink drop size at a pixel location adjacent to therespective pixel location but on a shifted raster.
 11. The methodaccording to claim 10 and wherein the array of nozzles are arranged inat least one row and including moving the ink jet print head in a fastscan direction perpendicular to the row of the array of nozzles; andmoving an ink jet print medium past the print head in a slow scandirection orthogonal to the fast scan direction.
 12. The method of claim11 and wherein ink drops are of a size such that when depositing a 2×2set or cluster of adjacent ink drops of the larger drop size in areference raster a gap is left in the center of the cluster, and an inkdrop of the smaller ink drop size when deposited on the shifted rasterin the center of the 2×2 set of adjacent ink drops covers the gap. 13.The method of claim 12 and wherein the nozzles when actuated are eachcapable of selectively producing ink drops of at least three drop sizesincluding the smaller ink drop size, the larger ink drop size and alargest ink drop size and wherein the size of the largest ink drop sizeis such that when depositing a 2×2 set or cluster of adjacent ink dropsof the largest ink drop size in a reference raster no gap is left in thecenter of the cluster, and in response to a maximum density signal for apixel location on the reference raster, the printer prints at therespective pixel location on the reference raster an ink drop of thelarger ink drop size and also prints an ink drop of the smaller ink dropsize at a pixel location adjacent to the respective pixel location buton the shifted raster so that the amount of ink deposited in a 2×2 setor cluster of adjacent ink drops of the larger drop size plus a smallerink drop in the center of the cluster comprises less ink than the 2×2set of adjacent ink drops of the largest drop size.
 14. The method ofclaim 13 and wherein an image processor determines from a sequence ofpixels that full coverage of an area is required and prints a 2×2 set orcluster of adjacent ink drops of the larger drop size plus an ink dropof the smaller ink drop size in the center of the cluster instead of the2×2 set of adjacent ink drops of the largest drop size.
 15. The methodof claim 12 and wherein the printer is adapted to print on differentrespective types of media and, in response to an input of type ofrespective media being printed on and a respective density at a pixellocation, the printer adjusts a signal to the nozzle to produce an inkdrop at the pixel location of a respective ink volume so that inresponse to the same density signal of a pixel received as an input theprinter prints an ink drop of different respective drop volume for eachof different types of media.
 16. The method of claim 12 and wherein theamount of ink deposited in a 2×2 set or cluster of adjacent ink drops ofthe larger ink drop size on the reference raster plus a smaller ink dropin the center of the cluster comprises less ink than would be providedby a 2×2 set of adjacent ink drops on the reference raster of the sameink type and same receiver media type but each of an ink drop sizesufficient to provide no gap in the center of the cluster.
 17. Themethod of claim 10 and wherein ink drops are of a size such that whendepositing a 2×2 set or cluster of adjacent ink drops of the larger dropsize in a reference raster a gap is left in the center of the cluster,and an ink drop of the smaller ink drop size when deposited on theshifted raster in the center of the 2×2 set of adjacent ink drops coversthe gap.
 18. The method of claim 17 and wherein the nozzles whenactuated are each capable of selectively producing ink drops of at leastthree drop sizes including the smaller ink drop size, the larger inkdrop size and a largest ink drop size and wherein the size of thelargest ink drop size is such that when depositing a 2×2 set or clusterof adjacent ink drops of the largest ink drop size in a reference rasterno gap is left in the center of the cluster, and in response to amaximum density signal for a pixel location on the reference raster, theprinter prints at a respective pixel location on the reference raster anink drop of the larger ink drop size and also prints an ink drop of thesmaller ink drop size at a pixel location adjacent to the respectivepixel location but on the shifted raster so that the amount of inkdeposited in a 2×2 set or cluster of adjacent ink drops of the largerink drop size plus a smaller ink drop in the center of the clustercomprises less ink than the 2×2 set of adjacent ink drops of the largestink drop size.
 19. The method of claim 17 and wherein the amount of inkdeposited in a 2×2 set or cluster of adjacent ink drops of the largerink drop size on the reference raster plus a smaller ink drop in thecenter of the cluster comprises less ink than would be provided by a 2×2set of adjacent ink drops on the reference raster of the same ink typeand same receiver media type but each of an ink drop size sufficient toprovide no gap in the center of the cluster.
 20. A method of printingwith an ink jet printer, comprising the steps of: providing an inkjetprint head having an array of nozzles, each nozzle being capable ofselectively producing when actuated an ink drop; and printing, inresponse to a signal calling for a maximum density value at a respectivepixel location, a larger ink dot at the respective pixel location on areference raster and a smaller ink dot at a pixel location adjacent tothe respective pixel location but on a shifted raster and wherein thelarger and smaller ink dots are of a respective size such that for a 2×2set or cluster of adjacent ink dots each of the larger ink dot size andeach on the adjacent pixel locations of the reference raster a gap isleft in the center of the cluster, and the smaller ink dot on theshifted raster in the center of the 2×2 set of adjacent larger ink dotsis of a size to cover the gap.
 21. The method of claim 20 and whereinthe larger dot on the reference raster is formed by multiple passes ofthe print head wherein an ink drop is deposited at the pixel location oneach of plural passes.
 22. The method of claim 21 and wherein thesmaller dot on the shifted raster is formed by multiple passes of theprint head wherein an ink drop is deposited on the shifted raster oneach of plural passes.
 23. The method of claim 20 and wherein the largerdot on the reference raster is formed in one set of plural passes of theprint head wherein an ink drop is deposited at the pixel location duringeach of the plural passes of the one set and only dots on the referenceraster are printed during said one set of plural passes and the smallerdot is formed during a second set of plural passes of the print headpass during which second set of plural passes of the print head printingof dots only on the shifted raster occurs.
 24. The method of claim 20and wherein the larger dot on the reference raster is formed in only asingle pass of the print head wherein a larger ink drop is deposited atthe pixel location during the single pass during which pass only dots onthe reference raster are printed and the smaller dot is formed by asmaller ink drop deposited during only another single pass of the printhead during which another single pass of the print head printing of dotsonly on the the shifted raster occurs.
 25. The method according to claim10 and wherein ink drops deposited on the reference raster are printedduring a pass of the print head over the receiver and only ink drops onthe reference raster are printed during said pass.